Time calculations

I would compute the difference using minutes because of the way JavaScirpt and computers divide in binary.

But since you have the difference in hours, your result field must have no formatting as the script will provide the formatted result.

// diffHrs = the difference in hours

// some debugging tracing
console.println("Difference in hours: " + diffHrs); // full decimal hours
console.println("Difference hours: " + Math.floor(diffHrs)); // hours truncated from difference
console.println("Difference minutes: " + diffHrs * 60); // computed minutes from full decimal hours
console.println("Difference minutes of hour: " + (diffHrs * 60) % 60); // minutes remainder (modulo) of full decimal hours
console.println("Difference minutes of hour rounded: " + Math.round((diffHrs * 60) % 60)); // rounded remainder minutes
// end debugging trace

// convert difference in hours into whole hours and minutes - minutes formatted with leading zero
event.value = Math.floor(diffHrs) + ":" + util.printf("%,002d", Math.round(diffHrs * 60) % 60);